1. Field of the Invention
The present invention relates to a piston assembly of a diesel engine, and more particularly, to a piston assembly including a piston crown formed of nodular graphite cast iron and having ribs therein
2. Description of the Related Art
In a general internal combustion engine, a piston transmits power to a connecting rod while vertically reciprocating along an inner wall of a cylinder during the intake-compression-combustion-exhaust cycle.
In the case of a high-performance engine, an upper structure (called a piston crown) of a piston including an combustion side and piston ring grooves is exposed to high-temperature, high-pressure combustion conditions during engine operation and is thus manufactured by a forging method for obtaining high-quality mechanical characteristics, while the rest of the piston (called a piston skirt), which is exposed to relatively less severe conditions, is formed of materials such as cast iron. However, if piston crowns are manufactured by a forging method in which a relatively expensive material is used, it is difficult to perform a machining process on the piston crowns. Therefore, the productivity of manufacturing is lowered, and the price of pistons is increased.
In addition, since such a piston crown is inefficiently cooled, the outer diameter of the piston crown may become larger than the inner diameter of a cylinder liner due to thermal expansion during engine operation, and in this case, the piston crown may melt and stick to the cylinder liner. To prevent this, the piston crown and the cylinder liner may be fitted with a large clearance therebetween. However, a large clearance lowers combustion efficiency and thus increases the generation of environmentally harmful pollutants such as fumes or NOx.
In the related art, one-piece pistons are formed by casting a piston crown and a piston skirt with the same material, such as cast iron, and some of such one-piece pistons have ribs formed therein. However, it is difficult to form oil galleries for storing oil in such one-piece pistons due to limitations of casting, and thus the one-piece pistons have low cooling efficiency, thereby generating large amounts of environmentally harmful pollutants like in the case of assembly type pistons formed of forged steel and cast iron. In addition, such one-piece pistons have low degrees of structural strength and are thus not suitable for high-power engines having high combustion pressure.
A method of forming ribs on a piston crown is disclosed in Korean Patent Laid-open Publication No. 2010-0127449. However, the method is for a structure in which a piston assembly is rotated and this application is to enhance the strength of a piston crown in a non-rotating structure is required.